The present invention relates generally to semiconductor switching devices and gate driving methods and, in particular, to driving switches comprising a normally-off semiconductor device and a normally-on high voltage wide bandgap semiconductor device in cascode arrangement.
The compound semiconductors, such as silicon carbide (SiC) and gallium nitride (GaN), have a bandgap or energy difference between the top of the valence band and the bottom of the conduction band typically greater than 2 electron volts, therefore these semiconductors are called wide bandgap semiconductors. The wide bandgap semiconductors have a much higher breakdown field than silicon; for example, the breakdown field of silicon carbide is about 3×106 volts per centimeter, which is about 10 times higher than the breakdown field of silicon. The properties of a wide bandgap and a high breakdown field allow the power devices made with wide bandgap semiconductors to block higher voltage with lower on-resistance, switch at higher frequency with higher efficiency, and operate at higher temperatures with less cooling requirements. These device characteristics are critical for implementing a high voltage, high temperature, high frequency and high power density power conversion system.
Tremendous efforts have been invested to develop SiC junction field effect transistors (JFETs), SiC metal-oxide-semiconductor field effect transistors (MOSFETs), and GaN high electron mobility transistors (HEMTs). Compared to SiC MOSFETs, SiC JFETs do not require a critical oxide film and therefore are free of oxide related issues of performance degradation and long-term reliability under high electric field and high junction temperature operation conditions. SiC JFETs are reliable at high temperature, and continue to undergo great advancement in specific on-resistance and switching figures of merit. By controlling the channel opening, a SiC JFET can be made to be normally-on or normally-off. A normally-on device is a device that is highly conductive, or in ON-state, when zero voltage is applied to its control terminal or gate. A normally-off device is a device that is highly resistive, or in OFF-state, when zero voltage is applied to its control terminal or gate. Normally-on SiC JFETs have demonstrated very low specific on-resistance. There is a penalty in specific on-resistance with normally-off SiC JFETs, and they require properly adjusted gate drives for optimal performance. As for GaN HEMTs, almost all of them are normally-on devices.
The best-performing SiC and GaN devices are normally-on devices. However, normally-on devices may produce a dangerous short circuit condition during the system startup or the gate drive supply failures, which make normally-on devices difficult to be applied in many power conversion applications. A method to overcome the challenge of using normally-on devices leverages the cascode concept disclosed by Baliga et al. in U.S. Pat. No. 4,663,547 entitled Composite Circuit for Power Semiconductor Switching, which comprises a low voltage normally-off MOSFET connected in series with a high voltage normally-on JFET and presents a normally-off operation mode when zero voltage is applied to its control terminal, the MOSFET gate. The advantages of the cascode device include normally-off operation mode, a built-in body diode with low forward voltage drop, and very low miller capacitance. The cascode device, as a composite circuit, contains many parasitic inductances from device bonding wires, package leads, and PCB traces. These parasitic inductances together with the capacitances of the two devices in the cascode might cause oscillations during switching process and result in instabilities under certain conditions. To ensure a reliable operation of the cascode, the instantaneous rate of voltage change over time (dv/dt) and the instantaneous rate of current change over time (di/dt) during switching process must be actively controlled. The MOSFET gate resistor and the parasitic inductance at the source of the cascode can be used to effectively control the di/dt and dv/dt of the turn-on process. However, complete control of the di/dt and dv/dt of the turn-off process of the cascode is more difficult than in the case of the turn-on process.
Various methods have been devised to control the switching process of the cascode. Rose, in U.S. patent application 20140027785 entitled Cascoded Semiconductor Devices, describes a method that uses a bootstrap capacitor connected between the gate of the high voltage normally-on device and the gate of the low voltage normally-off device in order to achieve an active control of both devices in a cascode circuit using a single gate driver. This method will develop a voltage drop across the bootstrap capacitor which reverses the bias of the gate-source junction of the normally-on device and results in a substantial increase in the on-resistance of the normally-on device.
Iwamura, in U.S. Pat. No. 8,487,667 entitled Hybrid Power Device, describes a method to control the cascode switching process with a resistor-capacitor-diode (RCD) network connected to the gate of the normally-on device. In order to attenuate the oscillations during the switching process to an acceptable level, this method reduces the switching speed significantly and increases the switching loss of the cascode circuit.
Friedrichs, in U.S. Pat. No. 7,777,553 entitled Simplified Switching Circuit, and Cilio, in U.S. Pat. No. 8,228,114 entitled Normally-off D-mode Driven Direct Drive Cascode, describe a direct drive method. In this method, the low voltage normally-off device is used as a protection device. In normal operation, the low voltage normally-off device is kept always on and the high voltage normally-on device is switched independently. During startup or a fault condition the normally-off device will be turned off, and the whole circuit will be turned off like a conventional cascode circuit. In this way, the direct drive cascode behaves like a stand-alone normally-on device and the limitations of the cascode circuit are avoided. However, this method needs a complicated gate driver to ensure proper safe operation of the cascode device.
Kanazawa et al., in U.S. Pat. Application No. 20130335134 entitled Semiconductor Device and System Using the Same, describes a method to control the cascode device. In this method, both normally-on and normally-off devices in the cascode are actively switched at the same time during the switching process and are actively maintained in OFF-state by their respective gate drive signal during OFF-state. With proper design of the delay time between the gate signals of the normally-on and normally-off devices, the cascode switching process will be very close to the switching process of a stand-alone normally-on device. However, the normally-on device is not suitable for reverse current conduction in this method because its gate terminal is kept at a low potential with respective to its source during OFF-state. The feature of the built-in body diode of the cascode device is not used. An additional freewheeling diode connected in parallel with the cascode device is required for the cascode device to conduct a reverse current.
Therefore, there is still a need for a cascode device having a simple gate drive circuit, controllable switching process, and a built-in body diode.